import sys

def resolve(in_):
    K, N, F = map(int, next(in_).split())
    _A = map(int, next(in_).split())
    A = sum(_A)

    ans = K * N - A
    if ans < 0:
        ans = -1
    return ans


def main():
    ans = resolve(sys.stdin)
    print(ans)


if __name__ == '__main__':
    main()